This invention is directed to electrosurgery and, in particular, to circuitry for monitoring patient return electrodes employed in such surgery.
One risk involved in electrosurgery is a burn under the patient return electrode. The most common conditions which are thought to lead to such a burn are:
(1) Tenting: Lifting of the return electrode from the patient due to patient movement or improper application. This situation may lead to a burn if the area of electrode-patient contact is significantly reduced.
(2) Incorrect Application Site: Application of a return electrode over a highly resistive body location (i.e. excessive adipose tissue, scar tissue, erythema or lesions, excessive hair) will lead to a greater, more rapid temperature increase. Or, if the electrode is not applied to the patient (i.e. electrode hangs freely or is attached to another surface), the patient is in risk of being burned by contact at an alternate return path such as the table or monitoring electrodes.
(3) Gel drying either due to premature opening of the electrode pouch or to use of an electrode which has exceeded the recommended shelf life.
Many monitor systems have been developed in the past, but most cannot directly guard against all three of the above listed situations. In order to protect against these potential hazard situations, the contact resistance of the return electrode to the patient itself should be monitored in addition to the continuity of the patient return circuit.
Safety circuitry is known whereby split (or double) patient electrodes are employed and a DC current (see German Pat. No. 1,139,927, published Nov. 22, 1962) or an AC current (see U.S. Pat. Nos. 3,933,157 and 4,200,104) is passed between the split electrodes to sense the contact resistance or impedance between the patient and the electrodes. U.S. Pat. No. 3,913,583 discloses circuitry for reducing the current passing through the patient depending upon the area of contact of the patient with a solid, patient plate, there being employed a saturable reactor in the output circuit, the impedance of which varies depending upon the sensed impedance of the contact area.
The above systems are subject to at least one or more of the following shortcomings: (a) lack of sensitivity or adaptiveness to different physiological characteristics of patients and (b) susceptibility to electrosurgical current interference when monitoring is continued during electrosurgical activation.